CVE-2026-42507

@@ -695,7 +695,7 @@ func TestHello(t *testing.T) {
 			err = c.Hello("customhost")
 		case 1:
 			err = c.StartTLS(nil)
-			if err.Error() == "502 Not implemented" {
+			if err.Error() == `502 "Not implemented"` {
 				err = nil
 			}
 		case 2:
@@ -953,8 +953,8 @@ func TestAuthFailed(t *testing.T) {
 
 	if err == nil {
 		t.Error("Auth: expected error; got none")
-	} else if err.Error() != "535 Invalid credentials\nplease see www.example.com" {
-		t.Errorf("Auth: got error: %v, want: %s", err, "535 Invalid credentials\nplease see www.example.com")
+	} else if err.Error() != `535 "Invalid credentials\nplease see www.example.com"` {
+		t.Errorf("Auth: got error: %v, want: %s", err, `535 "Invalid credentials\nplease see www.example.com"`)
 	}
 
 	bcmdbuf.Flush()

@@ -215,13 +215,13 @@ func (r *Reader) readCodeLine(expectCode int) (code int, continued bool, message
 
 func parseCodeLine(line string, expectCode int) (code int, continued bool, message string, err error) {
 	if len(line) < 4 || line[3] != ' ' && line[3] != '-' {
-		err = ProtocolError("short response: " + line)
+		err = ProtocolError(fmt.Sprintf("short response: %q", line))
 		return
 	}
 	continued = line[3] == '-'
 	code, err = strconv.Atoi(line[0:3])
 	if err != nil || code < 100 {
-		err = ProtocolError("invalid response code: " + line)
+		err = ProtocolError(fmt.Sprintf("invalid response code: %q", line))
 		return
 	}
 	message = line[4:]
@@ -253,7 +253,7 @@ func parseCodeLine(line string, expectCode int) (code int, continued bool, messa
 func (r *Reader) ReadCodeLine(expectCode int) (code int, message string, err error) {
 	code, continued, message, err := r.readCodeLine(expectCode)
 	if err == nil && continued {
-		err = ProtocolError("unexpected multi-line response: " + message)
+		err = ProtocolError(fmt.Sprintf("unexpected multi-line response: %q", message))
 	}
 	return
 }
@@ -541,7 +541,7 @@ func readMIMEHeader(r *Reader, maxMemory, maxHeaders int64) (MIMEHeader, error)
 		if err != nil {
 			return m, err
 		}
-		return m, ProtocolError("malformed MIME header initial line: " + string(line))
+		return m, ProtocolError(fmt.Sprintf("malformed MIME header initial line: %q", line))
 	}
 
 	for {
@@ -553,15 +553,15 @@ func readMIMEHeader(r *Reader, maxMemory, maxHeaders int64) (MIMEHeader, error)
 		// Key ends at first colon.
 		k, v, ok := bytes.Cut(kv, colon)
 		if !ok {
-			return m, ProtocolError("malformed MIME header line: " + string(kv))
+			return m, ProtocolError(fmt.Sprintf("malformed MIME header line: %q", kv))
 		}
 		key, ok := canonicalMIMEHeaderKey(k)
 		if !ok {
-			return m, ProtocolError("malformed MIME header line: " + string(kv))
+			return m, ProtocolError(fmt.Sprintf("malformed MIME header line: %q", kv))
 		}
 		for _, c := range v {
 			if !validHeaderValueByte(c) {
-				return m, ProtocolError("malformed MIME header line: " + string(kv))
+				return m, ProtocolError(fmt.Sprintf("malformed MIME header line: %q", kv))
 			}
 		}
 

@@ -411,6 +411,8 @@ func TestReadMultiLineError(t *testing.T) {
 		"Unexpected but legal text!\n" +
 		"5.1.1 https://support.google.com/mail/answer/6596 h20si25154304pfd.166 - gsmtp"
 
+	wantError := `550 "5.1.1 The email account that you tried to reach does not exist. Please try\n5.1.1 double-checking the recipient's email address for typos or\n5.1.1 unnecessary spaces. Learn more at\nUnexpected but legal text!\n5.1.1 https://support.google.com/mail/answer/6596 h20si25154304pfd.166 - gsmtp"`
+
 	code, msg, err := r.ReadResponse(250)
 	if err == nil {
 		t.Errorf("ReadResponse: no error, want error")
@@ -421,8 +423,8 @@ func TestReadMultiLineError(t *testing.T) {
 	if msg != wantMsg {
 		t.Errorf("ReadResponse: msg=%q, want %q", msg, wantMsg)
 	}
-	if err != nil && err.Error() != "550 "+wantMsg {
-		t.Errorf("ReadResponse: error=%q, want %q", err.Error(), "550 "+wantMsg)
+	if err != nil && err.Error() != wantError {
+		t.Errorf("ReadResponse: error=%q, want %q", err.Error(), wantError)
 	}
 }
 

@@ -41,7 +41,7 @@ type Error struct {
 }
 
 func (e *Error) Error() string {
-	return fmt.Sprintf("%03d %s", e.Code, e.Msg)
+	return fmt.Sprintf("%03d %q", e.Code, e.Msg)
 }
 
 // A ProtocolError describes a protocol violation such